The inventive subject matter disclosed herein relate generally to systems and processes using two or more channels of light each having different light attributes. In certain aspects, the inventive subject matter relates to the use of a continuous channel of light output and a superimposed pulsed channel in curing a target material. In such curing operations, a laminar flow system and process may be employed to deliver an inerting fluid or a reactive reagent across the surface of the target material to cause desired effects.
There are many applications in which light (i.e., any form of electromagnetic radiation) is used for industrial processing. For example, light is used in curing of adhesives, curing and drying of printing ink, curing of coatings, producing sterile reagents, and direct cleavage of biochemical bonds. In such processes, the target object being processed often has regions with distinctive physical and chemical properties. The target object may also be subject to environmental conditions that vary from one region to another, on or within the target object material. Accordingly, some of the material regions being processed may respond independently to different or variable light effects, such as different intensities, wavelength(s), total optical power emitted by the source, source coherence, radiance of the source (power/area×steradian), degree of collimation, and power stability. Unfortunately, known lighting systems have not been adequate at providing variable multi-attribute light effects that are tuned to specific material regions of a substrate.
For example, FIG. 1A is a graph showing a light output profile having a continuous power output signal (which corresponds to a radiance, which in turn correlates to an irradiance at the target substrate). Conventional light sources, including all commercial LED sources used for curing, are generally run in a continuous mode, at the light source's highest output. A continuous profile has been used to provide a good depth cure in a substrate. Unfortunately, this continuous profile, if at a power for a good depth cure, does not allow sufficient curing of curable material that may be in contact with an inhibitor.
For example, looking at FIG. 3, an irradiated target object 1 is shown. Although the target object 1 may be any irradiated object or substance, the following discussion, unless otherwise indicated, will use a UV curable ink on a printing substrate, such as a film or paper, as the example target object. The UV curable ink is a layer that is disposed over at least a portion of a substrate. The layer may be divided into two zones, Zone A and Zone B. Zone A has a surface on which the inhibitor, e.g., oxygen in air, can act. The inhibitor may also diffuse some depth into the layer to a boundary defining a first side of Zone B, where there is little or no diffusion of inhibitor. A second side of Zone B interfaces with substrate S. Therefore higher power levels are required to overcome the inhibition at the surface of Zone A and within it relative to a Zone B, which is not enriched with inhibitor. The curing operation must provide good curing at the Zone A, and operating in a continuous mode at the higher power to achieve a good cure in Zone A usually results in over-curing in Zone B, damage to the substrate or diminished lifetime of the light source.
The problems with existing lighting systems can be illustrated by looking, for example, at the case of curing or drying of acrylate inks in digital graphics. The result of such curing should be a dry and high gloss product. This has been achieved by dissipating a large amount of energy into the polymer ink formulations. Conventional mercury doped lamps and heaters are used as the light source. The print media typically consists of a variety of materials, some of which can be addressed by a higher energy method, while others of which (e.g., plastics like polyvinylchloride, polyethylene, and polypropylene, as well as various heat sensitive substrates) cannot be so addressed.
In another example case—DVD bonding—thermal loading of the polycarbonate disks is a particularly vexatious problem that may occur at different levels in the disc where materials, such as adhesive, interface with other materials. In conventional processes, thermal loading may cause deviations in the finished disc, e.g., in the axial, lateral and thickness dimensions. These kinds of deviations may detrimentally affect the read and write characteristics of the disc, reducing overall yield of the production line. The problems will only grow worse as the industry migrates towards lower initiator concentrations, and lower wavelengths (higher energy radiation) used to read and write on the DVDs.
Another challenge in DVD manufacturing is uniform curing across zones: one where there is an aerobic environment and the other where there is an anaerobic or less aerobic environment. The edge of a DVD is a particularly aerobic zone and therefore has required special processing parameters relative to the other zones of the disc.
FIG. 1B shows an implementation of a set of continuous power output signals where various LED array sources can be combined operating at different power levels or wavelengths shown in FIG. 1B. This continuous approach is restrictive and leads to inefficient energy use, heat loading and undesirable properties in the polymerized zones. In addition the lifetime of the source is decreased. Further, operation of the LEDs at the higher power outputs required for a good cure of some zones in a target object, such as aerobically inhibited surface zones, seriously shortens the life of the LEDs, making them impractical for use.
In summary, the continuous modes of operations shown in FIGS. 1A-1B are problematic at processing different zones in a single target object and wasteful of energy, considering that they must operate at outputs dictated by the more demanding zone of a given substrate, or have other drawbacks.
LEDs have also been used in a pulsed mode instead of a continuous mode. In a pulsed mode, the light output cycles between zero power output to a peak power output. Pulsing alone is not sufficient to achieve good overall curing of a substrate that has curable regions that independently react to light. For example, FIG. 2 shows a pulsing profile where the peak power may be sufficient to cure at the surface of, and within, Zone A, but does not yield a good cure at depth, with “depth” represented by Zone B in FIG. 3. Adhesion of materials at depth may be an objective of a curing process. Adhesion generally depends on the polymeric properties at the interface of the Zone B curable material and substrate (S) material, and pulsing that may afford curing in for Zone A cannot be controlled or optimized for good adhesion at the interface. Accordingly there has been a need for light systems that afford good curing at a surface zone and a depth zone.
Other approaches for overcoming the problem of edge curing include use of shutters and filters to vary the light exposure on a target object. But these systems are complicated, expensive, high-maintenance, and may require complex cooling systems.
In this regard, the prior art has attempted to differentially overcome aerobic inhibition by displacing air. For example, in DVD bonding this has been done by flooding the edge region of a disc with an inert gas, namely nitrogen. Unfortunately, flood inerting is inefficient and wasteful. (See UV-Sheetfed Drying Under Oxygen Reduced Conditions, by Joachim Jung, Peter Holl, Radtech 2004 Paper, IST Metz.) In addition to this, dynamic production processes, such as DVD production lines and printing-substrate belt-feed systems, are not amenable to isolation of the target object in a static reservoir of inerting gas. Therefore, these dynamic processes require a system where the gas that is used must be applied and purged from the point of treatment. Consequently, there are associated costs and potential hazards.
In view of the foregoing problems, there is a substantial need for improved lighting systems and methods that provide light to different zones of materials in a target object depending on the properties and requirements for each zone. There is also a need for improved systems and methods for introducing fluid flow to a zone of material for inerting the zone or for introducing a reactive reagent and thereby facilitate a desired photoreactive effect at the zone. There is also a need for such systems and methods to be efficient in terms of cost and effectiveness and relatively simple to implement and operate.